The invention relates to conveyorized transport systems, and more particularly to a computer controlled trolley and rail network providing flexible routing and reliable switching of trolleys as they travel to and from a plurality of work stations.
Conveyorized transport systems of the type with which this invention is concerned may, for example, be used in a garment making plant to carry workpieces to a series of work stations. In a factory without such a system a worker, in addition to performing a work operation on each workpiece, often has to punch a label attached to each workpiece to make a record of its status, sort the workpieces, and hand-carry them to various other work stations. These additional steps consume time and lead to other problems. For example, the time it takes a worker to perform various tasks is unpredictible and it is difficult for other plant personnel to keep track of the workpieces as they travel to and from the various stations, so production cannot be scheduled accurately. Also, it is common for operators to accumulate many workpieces before carrying them to a subsequent work station so there is much inventory tied up in the factory and with it, a high carrying cost. Furthermore, many times the operator will accumulate the workpieces in a pile in which case it is not easy to inspect them for quality control purposes.
There are conveyorized systems on the market today which utilize a main rail to guide workpiece carrying trolleys to subsidiary loops located along the main rail and each subsidiary loop leads to and from an associated work station. Between the main rail and each subsidiary loop are often two switches, one to divert a trolley and its workpieces from the main rail to the subsidiary loop and another to return the trolley from its subsidary loop to the main rail. These prior art switches use pivoting fingers having their pivot axes on the main rail, and are similar in principle to those used in railroad tracks, except that the pivoting fingers slope in the vertical direction when they switch to a subsidiary loop because the points on the subsidiary loops engaged by the fingers are not at the same height as the main rail. The main rail is higher than the entrance of each subsidiary loop so the pivoting finger leading to the entrance must dip to meet it, and the main rail is lower than the exit of each loop so the pivoting finger leading from the exit must rise to meet it. The reason that the entrance and exit of each subsidiary loop is displaced vertically from the level of the main rail is to allow a trolley to move by gravity as it switches between the main rail and a subsidiary loop. The pivoting finger used to divert a trolley to the subsidiary loop is located upstream relative to the main rail of the pivoting finger used for returning the trolley.
When a trolley is on the main rail a narrow pusher, carried by a propelling chain or cable moving parallel to the main rail, urges it along. When the trolley enters onto a pivoting finger angled toward a subsidiary loop, the pusher loses contact with the trolley and the trolley then moves under the force of gravity alone. This phase of trolley movement, from the main rail, onto a switch finger, and then onto the rail of the associated subsidiary loop, is very critical since the gravity biased movement of the trolley can be disrupted by a number of factors, such as wind from an open door or excess friction in the trolley, and lead to jamming of the system.
Once the trolley leaves the main rail and enters a subsidiary loop it continues under the force of gravity down a sloping portion of the loop until it reaches a gate and then awaits attention by an operator. The operator may subsequently perform a work operation on some or all of the workpieces associated with that trolley and when done, opens the gate and allows the trolley to proceed further along the subsidiary loop. Typically, this leads to an elevator which lifts the trolley and its workpiece to a level slightly above the height of the main rail. In coordination with this, the pivoting finger of an associated switch swings across and upward from its position in line with the main rail to connect the subsidiary loop to the main rail. The elevator then releases the trolley which rolls under the force of gravity onto the main rail. This phase of trolley movement is also critical and subject to the disruptions described above.
Because the discharge switch of a subsidiary loop is located downstream of its entry switch relative to the main rail, another subsidiary loop at which a subsequent work operation is to be performed on the same workpieces as worked on at a first loop must be located downstream of the first loop. That is, the prior art system does not permit subsidiary loops, at which successive operations are to be performed on its same workpiece, to be located directly across from one another on opposite sides of the main rail, and as a result subsidiary loops cannot be easily concentrated in a given floor space, and flexibility of movement of trolleys between various work stations is limited.
It is accordingly a general object of the invention to provide a conveyorized transport system which offers high flexibility of trolley routing.
Another object of the invention is to provide a reliable mechanism for use in a conveyorized transport system of the foregoing type for switching trolleys between a main rail and a subsidiary loop.
It is yet another object of the invention to provide such a switch which can transfer a trolley to an entrance of a subsidiary loop and receive a trolley from an exit of the subsidiary loop, the entrance being downstream of the exit relative to the general direction of trolley movement on the main rail, so that direct recirculation is feasible, subsidiary loops can be located directly across the main rail from each other to form pairs, and a trolley can be transported to each subsidiary loop of a given pair in either order.